Self-excited magnetron oscillator with frequency varying means



April 10, 1962 J. FORMAN 3,029,397

- SELF-EXCITED MAGNETRON oscILLAToR WITH FREQUENCY VARYING MEANS Filed April 20, 1959 3 Sheets-Sheet 1 l I l l l l 0 /0 Z0 30 40 50 60 70 ma /a /Al 6,4055' INVENTOE /V @FM4/v April 10, 1962 .1. FORMAN 3,029,397

sELF-EXCITED MAGNETRON OSCILLATOR WITH l FREQUENCY VARYING MEANS Flled Aprll 20, 1959 5 Sheets-Sheet 2 iii l I April 10, 1962 J FoRMAN 3,029,397

SELF-EXCITED MGNETRON OSCILLATOR WITH FREQUENCY VARYING MEANS 3 Sheets-Sheet 3 Filed April 20, 1959 INVENTOR. 2A/@Mmm United States Patent O fornia Filed Apr. 20, 1959, Ser. No. 807,590 Claims priority, application Great Britain Apr. 22, 1958 6 Claims. (Cl. 331-90) The present invention relates to squegging or quenched oscillators with particular reference to a circuit employing a split cylindrical anode magnetron. In the circuits to be described a four anode magnetron is shown although a two anode magnetron has also been found to be effective, and multi-anode types show certain advantages.

In accordance with the present invention I provide a circuit for producing continuous oscillations comprising a magnetron having a cathode and two or more separate anodes, a center tapped inductor connected between the anodes, with the cathode directly connected to the center tap, and means for providing an axial magnetic eld Within the magnetron. The magnetron produces a low frequency oscillatory voltage between said anodes, with the output being coupled from the inductor.

In the annexed drawings,

FIGURE l shows a schematic diagram of a known magnetron oscillator circuit.

FIGURE 2 shows a third schematic diagram of a magnetron circuit for producing oscillations in the absence of applied anode voltage.

FIGURE 3 shows a vgraph of frequency eld for the circuit of FIGURE 2.

FIGURE 4 shows a series of curves of the static anode current plotted against anode potential for a set of magnetic field strengths and shows the negative resistance in invariant position at Ea=zero volts.

FIGURE 5 shows a series of curves of the shift of the negative resistance point with different values of cathode temperature as `represented by dilferent heater potentials Eh.

FIGURE 6 shows a schematic circuit diagram of a preferred connection for a ten anode magnetron to obtain maximum sensitivity to magnetic eld strength variations.

VFIGURE 7 shows a schematic circuit diagram of a possible connection for producing oscillations but which is relatively insensitive to changes in magnetic iield strength.

In the circuit of FIGURE l comprising a standard split anode magnetron 1 having four cross connected segmental anodes, an oscillatory circuit 2 is connected between each pair of cross connected anodes. It is well known that if an axial magnetic eld whose strength is a well defined function of the anode potential is applied to the magnetron connected in such a circuit, it behaves as a negative resistance to the oscillatory circuit and oscillations are set up in the LC circuit. With normal emission from the cathode there will be space charge limited oscillations. Chokes 3 and 5 are provided to isolate the magnetron and oscillatory circuit from earth with respect to high frequency currents. D.C. power is provided by conventional means here shown as battery 4.

FIGURE 2 shows a circuit in which two pairs of cross connected anodes l10n, 11012, 111:1, and 111b are joined by a highly resistive iron-core inductance 112. Distributed capacity in the leads to the inductance is shown by condenser 118. The centre tap 113 of inductance 112 is connected to cathode 114. In this case the inductance was 300 henries and its total D.C. resistance was 8000 ohms. An output signal could be obtained by placing a against axial lographic trace taken at a successively greater magneticu eld strength. It may be seen that the position of the.

high impedance detector between points 116 and 117 or'' between point 115 and either end 116, or 117 of inductance 112. An axial iield is produced by a solenoid coil' 119 surrounding the magnetron and connected to apotential source 120 through a variable resistance 121.

Under the above conditions the V.H.F. oscillation is not generated and what is observed is a further type of i low frequency oscillation due to a negative resistance at zero applied anode potential and which appears to be a function of electron emission in the presence of a correct-` ly directed magnetic eld. That this negative'resistance exists, however may be seen from FIGURE 4 which represents the diode static characteristic of `a typical four segment magnetron, lwith all anodes tied together. A.

series of curves is shown, each one representing an oscilnegative resistance kink, designated 130, is invariant in position at whatever the strength of the magnetic ield and that it also makes its appearance at a lower iield strength than the negative resistance kink, designated 131,

which is the well-known magnetron negative resistancel characteristic, and whose position, along the voltage axis shifts progressively with increasing magnetic eld strength.

'Ihis position invariance to values of magnetic field is considered a new phenomenon and it has been found that the only manner in which the position of the negative resistance 30 kink can be shifted is by altering the temperature of the cathode, and thus is thoughtA to be an inherent property of cathodic emission in the presence of a magnetic field. This shift is shown in FIGURE 5, where each successive curve (all with a constant magnetic eld) represents a lower value of cathode temperature.

A curve of frequency against axial iield obtained for the circuit of FIGURE 2 is shown in FIGURE 3. The magnetron in the self-oscillating circuit of FIGURE 2 in addition to providing negative resistance may be re-l garded as a variable dynamic three-element capacitance. One plate of this capacitance is the electron plasma and the other two plates of the capacitance are formed by the two anodes. Since the radius of the electron plasma is controlled by the magnetic field, at the lower ylimit of the magnetic field strength, as shown by FIGURE 3, the

that a relatively large capacitance is provided. Under thiscondition the oscillating frequency is relatively low as shown by FIGURE 3. At the higher limit of the magnetic iield, the eifective capacitance is reduced and a higher frequency oscillation is the result. The sensitivity of oscillation frequency to axial iield changes was 7.4% per gauss, which is equivalent to .74 cycle/ second/gamma at 1 mc./s. The dynamic range of frequencies obtained is controlled by L, C and R values for the inductance 112 and in this case lay between 550 c./s. and 1350 c./s. per gauss.

Although the illustrations and embodiments described in this specification heretofore have represented the use of a four segment, split-cylindrical type of anode structure, as is well known in the magnetron art, equally successful operation has been obtained with multi-anode structures, having peripheral symmetry around the cathode but not necessarily in the form of a multi-split cylinder. Such a conguration is shown in FIGURE 6 which depicts a ten-anode structure, whose anodes 141 are U-shaped, with the base of the U directed towards the cathode. An output signal is obtained between terminals 142. and' 143. For maximum sensitivity to magnetic field variation alternate anodes are interconnected and from each veanode ring a lead is taken to a high valued center-tapped inductor 144 as depicted in FIGURE 2. That this is the preferred connection may be shown by the fact that if the anodes were to be connected as in FIGURE 7 into two parnted Apr. 1o, 1962 non-alternate groups of ve anodes, then the sensitivity to magnetic eld strength variations is considerably reduced.

A typical figure for sensitivity to magnetic field variations for the preferred anode connection of FIGURE 6 is 35% change of frequency per gauss. This change is positive and linear over a dynamic range of approximately plus or minus 1 gauss around a center frequency, the output being presented in digital form.

It is obvious that the number of anodes need not be limited to ten and that any even number of anodes may be employed. According to experimental evidence it appears that the greater the number, when connected in the preferred manner, the more sensitive is the circuit to magnetic field variations.

The shape of the anodes appear to be non-critical and an even number of rod-.like anodes or wires can produce the same results.

The capacitance 145 shown in FIGURES 6 and 7 may be either a physical capacitor or the stray capacitance of the magnetron, the inductor 144 and the leads.

Uses

The present invention will nd particular use as a means for measuring very small variations of magnetic ield because of the very great sensitivity of the frequency to changes of H. This is particularly true of the cathode temperature limited condition.

It may be shown that such a device as represented in FIGURES 2, 6 and 7 may be considered as means of converting heat energy in the cathode directly to alternating electrical energy across the output load circuit, since no anode potentials are applied. In the case of an electrical 1y heated cathode, as for instance by a D.C. current, the device may be regarded as a D.C. to A.C. converter. However it is not necessary that the cathode be heated by electrical means, since radiative or conductive thermal energy may effect the heating of the cathode; in which case the device may be regarded as a means of converting thermal energy to alternating electrical energy. Although thermionic emitters have been referred to in the specication, any form of electron emitter may be employed, such as photoemitter, or substances emitting electrons due to impinging radiation.

-I claim:

1. A self-excited oscillator for generating a low frequency alternating voltage output signal across a pair of terminals comprising an electron discharge device including a linear emitter, means for heating the emitter to drive oi electrons from the surface, and at least a pair of anodes positioned concentrically about the linear emitter, a highly inductive center-tapped impedance connected between said pair of anodes, means for direct coupling of the center-tap of the impedance to the emitter with a very small potential difference existing across the direct coupling means, means for producing a magnetic eld in the region of the emitter having a major component of the ux aligned with the linear emitter, and means coupled across the impedance for connecting the alternating signal derived from the impedance to the output terminals.

2. Apparatus as defined in claim l wherein the impedance includes a transformer having a center-tapped primary winding connected between the anodes, and a secondary winding coupled to the output terminals.

3. A self-oscillating circuit for generating an adjustable frequency alternating voltage between a pair of output terminals, said circuit comprising a center tapped inductance coil coupled between the output terminals, a vacuum tube including an electron emitting cathode and a plurality of anodes connected in two groups, each group having at least one anode, the anodes being spaced from the cathode, the cathode being directly connected to the center tap of the inductance coil and the two groups of anodes being coupled respectively to the two ends of the inductance coil, means for producing a magnetic iield in the region between the cathode and the anodes such that oscil lations are sustained across the inductance coil to provide an output signal at the output terminals, and means for varying the field strength to change the frequency of the output signal.

4. Apparatus as detined in claim 3 wherein the inductor has an iron-core to provide high inductance.

5. Apparatus as defined in claim 4 wherein the inductor is highly resistive.

6. Apparatus as defined in claim 3 wherein said means for coupling an output signal includes a secondary winding inductively coupled to the inductor.

References Cited in the file of this patent UNITED STATES PATENTS 2,027,919 Lindenblad Jan. 14, 1936 2,064,012 Kilgore Dec. l5, 1936 2,121,067 Brown et al. June 2l, 1938 2,293,798 Braden Aug. 25, 1942 2,391,545 Chodorow Dec. 25, 1945 2,590,612 Hansell Mar. 25, 1952 

